Electric microphone



Mrch 15, 1932. P. D. TYERS 1,849,891

ELECTRIC MICROPHONE Filed May 28, 1928 microphone consists essentially of cies.

Patented Mar. 15, 1932 UNITED STATES-- PATENT" OFFlCE- PAUL DOUGLAS TYERS, or wA'rroRn, ENGLAND, assumes no ren'nivrc ELECTRIC,

COMPANY LIMITED, OF LONDON, ENGLAND nnnornio Mrcnornonn Application filed May 28, 1928, Serial No. 281,332, and in Great Britain une 2, 1927. '7

This invention relates to electric micro-' phones and has for its object the provision of a microphone which shall have a fairly even response over a wide range of frequen- A further feature of this invention is that a'microphonemay be constructed so as to be comparatively robust without departmg from a good response characteristic over a wide range. The well known carbon compression efiect is utilized for the microphonic property of the microphone. A carbon 21. conducting'back plate,.a layer of granules and a carbon diaphragm or the like. The response of such an instrument is very unequal and it is quite useless for accurate sound reproduction; Another type of carbon microphone which has an even responsehas been developed, and consists essentially of a layer of carbon granules, usually held in position by means of a sheet of rubber. The current path is along the length ofthe layer, that is, substantially at right angles to the normal direction of the sound waves. layer is provided with two conductors, usually in the form of carbon rods, at each end of thelayer, connection being made to these rods. This type of microphone has been described in some detailin order that it may be clearly differentiated from the present invention. t

The principle employed in the carbon microphoneconstructed according to this invention, is that of the usual type, in which the current path is substantially in the same plane as the sound waves. In other words the current passes between a diaphragm and a conducting back plate at-right angles to the plane of the carbon layer.

. According to this invention a microphone is constructed substantially as follows. The diaphragm is of a composite nature and consists-of a conducting portion and a support ing portion. The conducting portion isa substantially uniformand even layer ofextreme thinness of metal which does not oxidize easily, such as gold. This is attached to the supporting portion,',which is an area of rubber or. similar material, the thickness of which is preferably not morethan one or two The carbon millimetres; The metallicized diaphragm may be produced in several ways, but whatever method is. adopteditfalls into one of two processes, electrical deposition or cementing. lVhen the metal is electrolytically deposited the rubber may becoated' with carbon to render it suificiently conductive for deposition to occur. The actualmethod of coating however, is not a feature of this inventionv and it may beaccomplished by any means known totlieartf P f r When the rubber orlike material is metallicized by I cementing jcertain methodsare found necessary and these constitute features of this invention. The metal such as gold is beaten into the form of leaf in known manner and it isplaced on a plane surface in an atmosphere free from heavy air currents. The rubber is then given a coating of adhesive and when it is tacky it is placed on the metal leaf which will then adhere firmly to it. It is a feature of this process that'whatever adhesive may be used, the solvent has no appreciable action on the rubber or like material, and that the adhesive when dry may be of a flexible nature. It is also necessary to use the minimum quantity so that it has no appreciable effect upon the mechanical properties of the diaphragm. Byfwayflof example may be mentioned water soluble gums which when set remain flexible for considerable periods. Another method which may be used is that of spraying the rubber with atomized aoetoneor similar solyent containing a small amount of celluloid orcellulose acetate. It is a. further feature of this invention that-the coating operation be carried out when the rubber is attached to the supporting frame or like constituting part of the microphone. I If this is not done,;the metal layer may be broken when the rubber is fixed tojits sup-i porting frame which will be referred toher'ee inafter. c v a The diaphragm hereinbefore particularly described is attached to a frame or support either by clamping or by means of any suitable adhesive. It is important that the rub-- her is only slightly stretched, the stretching employed being that suflicient to remove any sagging when the frame is held vertically.

a block of fairly non-resonant substance such I as marble, but it is not desired to limit this invention to the use of this or like substance If the housin is of non-conducting-material V such as marb c it is necessary to renderthe metal. Thefreeendiofthis' foil strip isthen granule recess conducting at theback, by inserting a carbon or metal plate. When the granule housingis -of c onducting material it is preferable to insulate the sides of the recess in any jconvenient manner in order to obtain a uniform current distribution through the carbon layer.

Connection is 'made to the'diaphragm in the following manner. The front of the granule housing around the edge of the recess iscoated with one or more layers of soft metalfoil, such as'lead or tin,,the conducting edge thus formed being in intimate contact with a strip ofsomewhatlthicker foil of any connected to a terminal. Anotherlterminal is connected inany desired manner with the back conducting plate. When the granule housing is ofconducting material the foil edgeof the granule recessimust, of course, be insulated from it, preferably by inserting an insulating gasket or washer. The prepared diaphragmattached to a frame is placed in front of the recess and the metallicized edge beds on to'thesoft foil, thereby establishing good contact with it. In order to introduce the granules, the recess is provided with a filling plug or the like. When 7 the granule housing is of insulating material the terminals may be mo unted directly upon it. When, however, the material is conductiVet-he terminals must be insulated there from. The leads between the respective elements of the 'microphone and the terminals are taken through holes or tunnels suitably positioned'in the granule housing. When the housing is conductive one terminal may if desired be connected directly to the housing. When extreme p'urityin reproduction is desired it is necessary to eliminate every source of inherent mechanicalresonance'as far as possibleand a feature of this invention lies in making the terminals of large diameter compared with their length and placing them as far as possible out of direct sound waves impinging upon the diaphragm. For example the "terminals may be placed at the back of the granule housing or in suitable recesses at the sides of the housing, so 7 that they are not subjected to direct sound waves; r

In order to protect the diaphragm from mechanic'al'injury the supporting frame may 'metallic or non-metallic.

microphone intended for great purity, the gauze is of muslin or like material free from natural resonances. The mechanical construction of the-granule housing, recess, and diaphragm frame is not of great importance and does not form an essential feature of this invention and it may be modified with-V out departing fro nthe scope 'of the invention. The construction hereinbefore described is given as one convenient manner of carrying the invention into effect, and should be readily understood by those skilled in the art.

The dimensions of the granule recess and therefore the granule layer contained there in and-the size of the granules are important factors determining the performance of the microphone, and the proportioning of these parameters in conjunction with the composite diaphragm hereinbefore described forms one of the essential features of this in vention. The effect of the depth of layer and size of granule is well known. Briefly, however, the thicker the layer the less Will be the total response to the high frequencies, the inverse also being true. It is also known that if the size ofthe granulesis large therewill be appreciable air space between them and therefore the absorption Will'vary at different frequencies. It is customary to obtain "a substantially equal response over a desired frequency range by using a mixture of gran- I ules of various sizes and it willbe obvious that s1m1lar frequency responses can be ob mixtures of In the case of a" greater will be the head on the diaphragm, f

the term head being used in a hydrostatic sense. If this head is great, the granules will tend to pack and the microphone will be liable to go out of adjustment when subjected to strong mechanical shock." A feature of a microphone constructed according to this invention is that by using a small volume of granules and therefore a thin layer, comparative robustnesscan'be obtained, and at the same time an even frequency: characteristic can be obtained, by suitably adjusting the size and mixture of the granules according to known methods and practice. It will be obviousthat when the layer is very thin, the ohmic resistancewill be low and, accordingly the microphone may be worked at fractio n'al voltage if desired.

the use of a plurality of microphones of the type described the microphoneunits being iic Another feature of this invention lies'in naaa aar phone of considerable sensitivity. When a composite microphone is employed, the various units maybe so constructed thatsome, for example, respondespecially to the higher frequencies, while othersrespond'espeoially to the lower frequencies. In the case of a composite. microphone the various granule layers and diaphragms maybe mounted in one granule housing, or they may be provided with separate housings.

In order that this invention may be clearly understood several methods of applying its microphone construction are shown in the accompanying drawings. Figure 1, shows one form of suitable granule housing, while Figure 2, shows a cross section through such a housing with a diaphragm attached. Figure 3 shows the invention applied to another form of construction, while Figure 4, shows a composite microphone.

Referring to Figs. 1 and 2, the granule housing comprises a block of insulating material of the type described 1, provided with a recess 2, containing a conducting plate 3. The edge of the recess is coated with foil at 4, and holes 5 and '6 are provided for heavier foil leads to the foil edge 4, and the plate 3, respective. A filling orifice 7 closed by a plug 8 communicates with the granule recess 2. The housing is provided with holes 9 adapted to take bolts or the like 11, for the purpose of retaining the diaphragm. Holes 10 are shown for the purpose of retaining bolts used to locate the plate 3, in the recess 2. The diaphragm 12, fixed to a frame 13, is protected by a gauze area 14, the whole being fixed in position by a frame ring 15, and the bolts 11. The leads and terminals are omitted for clearness and also the section lining of the housing in Fig. 2.

Figure 3, shows a more compact form in which the granule housing 16, is metallic and is provided with a granule recess 17, and an insulating gasket of the shape indicated 18. This is coated with metal foil 19 making contact with the diaphragm 20, attached to the frame 21. The diaphragm is protected by a gauze area 22 the whole being held in position by a frame ring 23, and bolts 24. A filling orifice is closed by a plug 26. Terminals are provided at 27 one being attached directly to the granule housing and the other to an insulating plate 28, held in position by bolts 29. A hole 30, is provided in the granule housing for a heavy foil or other lead between the appropriate terminal and the foil surface 19. The section lines on the granule housing are again omitted for clearness.

Figure 4, shows a composite microphone comprising a large granule housing 31, provided with a plurality of microphone elements 32. Terminals 34, are fixed to a plate of insulating material 33 carried at the back of the granule housing.

- It is. to .be understood that this invention is not limited to any particular formiof mechanical construction and those hereinbefore described are givenonly as methods of carrying'the invention into eflect. Both circular and squarediaphragms have been shown,-but other shapes maybe used if desired. Similarly known .methods 10f microphone construction may be used: in conjunction with this invention without departing from its scope. ,1, 7 It is "particularly not desired to limit: this invention to specific sizes and mixtures of granules :and thickness of layers as the inethv od of adjusting them vforany desired'frequency response is well known to the art, and these factors are naturally determined by the particular purpose for .whichthe microphone is required. A feature of this inventionhowev'er, lies in the: use ofa' very thin layer with suitably proportioned granules for the response desired, when the mi crophone is to be of a robust natured Hith ertoxmicrophones have employed comparatively thin layers of the order of .3 .millis metres, and'whenthe area is appreciable the head is large. :Athin'layer according to this invention is intended to mean one of the order of less than .3 millimetres. V Itfis clearly tobe understood, however, that comparatively thick layers may be used in microphones constructed'according to this invention. 5 What I claim and desire to secure by Let'-' ters Patent is'zl i 1. An electric :microphone comprising a quantity of conductive. granular material, a diaphragm including a sheet of'rubberland a thin metal layer-intimately attached to one another, means'for holding said granular material with a part thereof resting against the metal layer of saiddiaphragmand for supporting said diaphragm with the sheet of rubber directlyexposed to the outer atmosphereand means for connecting the two arms of a circuit respectively to said metal layer and to a part of the granular materialremote from said di aphragm whereby, as said diaphragm vibrates the electrical resistance between said two arms varies.

2. An electric microphone comprising a quantity of conductive granular material, a diaphragm including a sheet ofrubber and a thin metal layer intimately attached toione 0 another, an open ended housing of insulating material having its open end closed bymeans of said diaphragm with'the m'etallayer Linwards, whereby an enclosed space islconstituted containingsaid granular material, a seating forsaid diaphragm of conductivematerial carriedby saidhousing and means for connecting two arms of acircuit'respectively to said seating 'andto apart ofthe granular material remote from the diaphragm.

3.1.An electric. microphone comprising 9. H30

lar material, a seating for said diaphragm ofconductive material carried by saidhous- I ing and meansfor connectingthe two arms of a circuit respectively to said seating and tea part of the granular material'remote fromthe diaphragm. I c A 4:. An electric microphone comprisin r a quantity of conductive granular materia ,a diaphragm including a sheet of rubber and a thin metal layer intimately attached to one another, an open ended housing of insulating material having its open end closed by means of. said diaphragm with the metal layer inwards, whereby anenclosed space in constituted containing said; granular material said housing-being soformed that said space is shallow in a sense at right angles to said diaphragm, a seating for said diaphragm of conductive material carried by said housing, a plateof conductive material located in said housing remote from said diaphragm and means for connecting the two arms-of a circuit respectivelyfto said seating and said plate.

5. An electric microphone comprising a r ing'remote from said diaphragm, means for connectingthe two arms of a circuit respective to said seating and said plate and a sheet of non-resonant gauze-like material located in frontof saididiaphragm.

In witness whereof I afiix my signature.

PAUL DOUGLAS TYERS.

quantity of conductive granular material, a i I diaphragm including a sheet of rubber and a thin metal layer intimately attached to one another, an open endedqhousing having its open end closed by means of said diaphragm with-the metal layer inwards, whereby an enclosed space is constituted containing said" granular material, said "housing being so formed that said space is shallow in a sense at right angles to said diaphragm, a seating for said diaphragm. of conductive material carried bysaidhousing, a plate of conductive material locatedin said housing remote from said diaphragm, terminals connected respectively to said seating and said plate,

said terminals being out of the direct path of the sound waves influencing said diaphragm and being of large diameter compared'to their. length. v 1 e 6. An electric microphone comprising a quantity of conductive granular material, a diaphragm including a sheet of rubber and a thin metal layer intimately attached to 'oneanother, an openended housing of insulating material having its open end closed by means ofsaid diaphragm with the metal layerinwards, whereby an enclosed space is constituted containing saidgranular; mate- 'rial,.a seating for said diaphragm of conductive material carried by said housing, a plate a of conductive material located in said hous- 

